The present invention is directed to electrochemical devices, and more specifically is directed to electrochemical devices having oxygen electrodes which contain specified pyrochlore type compound electrocatalyst materials including specified lead-rich pyrochlore compounds.
A number of various types of electrochemical devices have been developed over the past few years for the production of electrical energy by electrochemical reaction and obversely for the consumption of electrical energy to effectuate electrochemical reactions. Many of these devices rely upon a reaction involving oxygen (or air) as part of the mechanism to accomplish the desired result. For example, such devices may contain oxygen electrodes which are oxygen reducing cathodes in which oxygen is catalytically electroreduced. Alternatively, such devices may contain oxygen electrodes which catalyze the evolution of oxygen from water. In general, these electrodes are known in the art as oxygen electrodes. Thus, metal-oxygen batteries, metal-air batteries, fuel cells, electrolyzers, metal electrowinning devices, etc., are among the well-known electrochemical devices which may contain oxygen electrodes. Typically, such devices contain electrocatalyst materials at one or more of their electrodes and precious metals, such as platinum (on carbon support) and silver (on carbon and other supports), are frequently employed as electrocatalysts.
In addition, various electrocatalytic alloys, compounds and compound mixtures have been developed for these electrochemical devices to achieve more desirable systems. For example, U.S. Pat. No. 3,536,533 (Kitamura) describes the use of an alloy of gold, silver, palladium and at least one of platinum, rhodium and ruthenium as a fuel cell electrode electrocatalyst, and U.S. Pat. No. 3,305,402 (Jones et al) describes the use of a combination of platinum and ruthenium oxides as an electrocatalyst. However, both references describe these catalysts as fuel cell anode (or fuel oxidation) catalysts. O'Grady et al, Technical Report No. 37, "Ruthenium Oxide Catalysts for the Oxygen Electrode," Contract No. N0014-67-A-0404-0006 (AD-779-899) Office of Naval Research, May, 1974 (National Technical Information Service) describes the use of ruthenium oxide as an electrochemical catalyst for both the generation of oxygen and the reduction of oxygen. U.S. Pat. No. 3,405,010 (Kordesch et al) teaches that spinel type electrode catalysts have been found to produce better activation of the electrode and improved electrolyte repellency of the electrode by the inclusion of ruthenium.
The foregoing prior art describes various types of electrodes including those which utilize iridium and/or ruthenium-containing catalysts. However, none of these references teaches or renders obvious the electrochemical devices of the present invention having the specified pyrochlore type compounds as the electrocatalysts.
Heretofore, many pyrochlore compounds such as the pyrochlore compounds Pb.sub.2 Ru.sub.2 O.sub.7-y (lattice parameter of 10.253A), Pb.sub.2 Ir.sub.2 O.sub.7-y (lattice parameter of 10.271A), Bi.sub.2 Ir.sub.2 O.sub.7-y, Bi.sub.2 Rh.sub.2 O.sub.7-y, Pb.sub.2 Rh.sub.2 O.sub.7-y, Pb.sub.2 Pt.sub.2 O.sub.7-y and Cd.sub.2 Re.sub.2 O.sub.7-y, commonly referred to as lead ruthenate, lead iridate, bismuth iridate, bismuth rhodate, lead rhodate, lead platinate and cadmium rhenate, respectively, have been known. For example, Longo, Raccah and Goodenough, Mat. Res. Bull., Vol. 4, pp. 191-202, (1969) have described the compounds Pb.sub.2 Ru.sub.2 O.sub.7-y and Pb.sub.2 Ir.sub.2 O.sub.7-y and their preparation at elevated temperatures which are in excess of 700.degree. C. Sleight, Mat. Res. Bull., Vol. 6, p. 775 (1971) has also described the compounds Pb.sub.2 Ru.sub.2 O.sub.7-y and Pb.sub.2 Ir.sub.2 O.sub.7-y (including the pyrochlore compound Pb.sub.2 Ru.sub.2 O.sub.6.5 having a lattice parameter of 10.271A) and their preparation at 700.degree. C. and 3000 atmospheres of pressure. U.S. Pat. No. 3,682,840 (Van Loan) describes the preparation of lead ruthenate at temperatures of 800.degree. C. and higher. These references do not teach that lead-rich compounds used in the present invention exist or that they may be prepared by solid state techniques at temperatures below about 600.degree. C. in an oxygen-containing environment. Further, they do not teach that the known pyrochlore compounds may be useful as electrocatalysts in electrochemical devices such as in the present invention.
U.S. Pat. Nos. 3,769,382 (Kuo et al) and 3,951,672 (Langley et al) both disclose the preparation of lead ruthenate and lead iridate using various techniques at temperatures of at least about 600.degree. C., and preferably at higher temperatures. However, these references fail to recognize that the lead-rich pyrochlores used in the present invention are obtained at generally lower temperatures or that such pyrochlores have improved physical properties. Further, these references also fail to teach or render obvious the use of any pyrochlore compounds as electrocatalysts in electrochemical devices such as in the present invention.
Bouchard and Gillson, Mat. Res. Bull., Vol. 6, pp. 669-680 (1971) describe Bi.sub.2 Ru.sub.2 O.sub.7 and Bi.sub.2 Ir.sub.2 O.sub.7 preparation and properties, including the fact that these compounds have high conductivity and small Seebeck coefficients. However, there is no teaching that these compounds are useful electrocatalysts in electrochemical devices. Derwent's Basic Abstract Journal, Section E, Chemdoc, Week No. Y25, Abstract No. 320 (August 17, 1977), Derwent Accession No. 44866Y/25 describes electrodes for electrolysis of alkaline and carbonate solutions which comprise nickel-plated steel strips coated with high conductivity layers containing Cd.sub.2 Re.sub.2 O.sub.7, Pb.sub.2 Re.sub.2 O.sub.7-y or Ni.sub.2 Re.sub.2 O.sub.7. These compounds are prepared by impregnating perrhenic acid and a metal nitrate such as Cd nitrate onto a nickel strip and baking at 350.degree. C. However, these compounds are all rhenates and are not included in the present invention electrochemical devices due to their relative inferiority because of their strong tendency to oxidize to perrhenates (Re.sup.7+) and to dissolve in typical electrolyte systems. National Bureau of Standards, Wash. D.C. Inst. for Mat. Research, Abstract of Rept. No. NBSIR-75-742 (1975) describes the use of mixed oxides as oxygen-reducing electrocatalysts in acid fuel cells, including the use of barium ruthenate. However, of all materials suggested for such electrocatalysts, none are of the pyrochlore type structure used in the electrochemical devices of the present invention. Thus, it should be noted that, for example, barium ruthenate is not a pyrochlore whereas lead ruthenate, used in the present invention, is a pyrochlore.
In summary, there exists a formidable body of prior art describing the existence of various pyrochlores, their potential uses including uses as dielectric materials, and describing various metals and metal oxides as electrocatalyst materials. Notwithstanding such prior art, there is no suggestion or teaching that (a) the lead-rich pyrochlore compounds used in the present invention even exist, or that (b) the specific pyrochlores used in the present invention, including known pyrochlores, may be useful electrocatalyst materials in electrochemical devices.